1. Field of the Invention
The present invention relates in general to a shift control apparatus for controlling shifting actions of an automatic transmission, and more particularly, to improvements of a shift control apparatus for controlling shifting actions of an automatic transmission each of which is achieved by engaging and releasing respective two hydraulically operated frictional coupling devices of the automatic transmission.
2. Discussion of Related Art
One example of an automatic transmission for a vehicle is a planetary gear type transmission including hydraulically operated frictional coupling devices each having a hydraulic cylinder which is supplied with a working fluid when the frictional coupling device is engaged. For controlling shifting actions of such a planetary gear type automatic transmission, there is known a shift control apparatus comprising (a) a transient-hydraulic-pressure control device operable to control a transient value of a hydraulic pressure in the hydraulic cylinder of the frictional coupling device according to an electric signal, when the frictional coupling device is released, and (b) an accumulator connected to the hydraulic cylinder and operable to control a transient value of the hydraulic pressure in the hydraulic cylinder, when the frictional coupling device is engaged with the working fluid being supplied to the hydraulic cylinder, the shift control apparatus being operable to control shifting actions of the automatic transmission by engaging and releasing actions of the frictional coupling device. An example of this type of shift control apparatus is disclosed in Instruction Manual for New Model “LAND CRUISER 100”, published on Aug. 5, 2002 by Toyota Jidosha Kabushiki Kaisha.
JP-2000-205403A, JP-11-22817A, JP-6-109119A and JP-9-39859A disclose related techniques in the field of a shift control apparatus for an automatic transmission, which is as described above.
In the shift control apparatus of the type described above, a degree of smoothness of an engaging action of the frictional coupling device with a predetermined inertial characteristic increases with an increase in the capacity of the accumulator used, so that the amount of a shifting shock of the automatic transmission upon engagement of the frictional coupling device can be reduced by increasing the capacity of the accumulator. On the other hand, an increase in the capacity of the accumulator results in an increase in the length of time required for the pressurized working fluid to be fully discharged from the accumulator, upon releasing of the frictional coupling device, leading to undesirable deterioration of an operating response of the transient-hydraulic-pressure control device, since adequate control of the hydraulic pressure of the frictional coupling device by the transient-hydraulic-pressure control device is possible only after the full discharging of the working fluid from the accumulator. Thus, an increase in the accumulator capacity provides an advantage on one hand but at the same time suffers from a problem, in controlling the shifting actions of the automatic transmission.